Powder metal technology is well known to the persons skilled in the art and generally comprises the formation of metal powders which are compacted and then subjected to an elevated temperature so as to produce a sintered article.
Typically the percentage of carbon steel lies in the range of up to 0.8% C. Ultra high carbon steels generally speaking have carbon contents between 0.8% to 2.0% carbon.
It is known that tensile ductility decreases with an increase in carbon content and accordingly ultra high carbon steels have historically been considered too brittle to be widely utilized. However, the strengthening effect of carbon in steels is well understood.
Ultra high carbon steels have been produced as disclosed in U.S. Pat. No. 3,951,697 as well as in the article by D. R. Lesver, CKSYNA. Goldberg, J. Wadsworth and OD SHERBY, entitled "The Case for Ultra High Carbon Steels As Structural Materials" appearing in the Journal of Minerals, Metals and Materials St., August 1993.
Generally speaking the brittleness of such high carbon steels results from carbides which precipitate during the austentite to ferrite transformation during cooling. Moreover the reference to spheroidization refers to any thermo mechanical process that produces a rounded or globular form of carbide. Spheroidization is the process of heat treatment that changes embrittling grain boundary carbide and other angular carbides into rounded or globular form. In the prior art, the spheroidization process was time consuming and uneconomical as the carbides transform to a rounded form only very slowly. Typically spherodization requires long soak times of several hours at temperature. Mechanical working at elevated temperature has been used to speed up the spherodization process. However this adds costs and is only possible for relatively simple shapes.
The applicant herein has improved the prior art process of producing sintered metal articles having relatively high densities that include heat treatment steps which rapidly spherodize embrittling carbides. For example, applicant obtained U.S. Pat. No. 5,516,483 which relates to a process of forming a sintered article of powder metal comprising blending carbon and ferro alloys, lubricant with iron powder then high temperature sintering the article in a reducing atmosphere then spherodizing the sintered ultra high carbon steel. The use of silicon is disclosed but added as a ferro alloy namely ferro silicon to the iron powder.
Moreover applicant has obtained U.S. Pat. No. 5,552,109 which relates to a high density sintered alloy and spheroidization method utilizing pre-alloyed powders with for example 0.85% Mo in the pre-alloyed form blended with graphite and lubricant. U. S. Pat. No. 5,552,109 exhibits excellent results utilizing a spheroidization method where cooling may occur by oil quenching.
It is an object of this invention to provide an improved powder metal method whereby high density products are produced by spheroidizing with the rapid cooling utilizing a fan in a reducing or neutral atmosphere.
Although U.S. Pat. No. 5,516,483 taught the use of silicon such silicon was added in the form of ferro alloy namely ferro silicon. Generally speaking graphitization elements such as nickel and silicon (other than as trace elements) are to be avoided as taught in U.S. Pat. No. 5,641,922. Moreover if silicon is added as elemental silicon it tends to oxidize which is detrimental to the sintered powder metal article in both fatigue or endurance properties.
Silicon has been added to copper based sintering as shown in U.S. Pat. No. 2,372,203 as well as for cutting tools as shown in U.S. Pat. No. 4,011,108 and also in aluminium alloys as taught by U.S. Pat. No. 4,711,823.
It is a further object of this invention to provide simplified apparatus and method for producing sintered powder metal articles.